I think on a more general level people might post speculation about this political group, that professional group, some religious group, etc. and never once consider that groups are made of individuals. When you suggest any group might think/believe/do X then you are implicating individuals as well. When you indtroduce your specualtions into a public forum, especially one as vast as fb then there is a chance that people who knw or even are some of those individuals will see and take offense at the implications. For this reason when there is evidence disproving, as in OP's case, I don't think it has to be done in a PM. And I don't think the person posting should be hurt or suprised by the fallout. And the reality is that while this is one of the most recent and emotional examples, there ae several religious groups, political groups, professional groups, social groups, etc. who have had people "wonder" what they might be capable of. Introducing those suggestions are incredibly hurtful to individuals as well as groups.

Why not just ask if they believe it? Doesn't have to be confrontational, just "Saw you posted X. What's up?"

That just seems way too ambiguous to me. I think being direct is a better approach (if one is so inclined).

My wording was lousy, but my point was, why go straight to defriending or hiding or whatever when asking would get the answer the OP is looking for.

Whether the niece is serious or not, giving this particular hoax (or ones like it) any attention other than to decry them would cause me to defriend.

Okay. I just prefer not to go nuclear bomb until I know it's warranted.

I don't see defriending as nuclear bomb.

For me, defriending is kind of a nuclear bomb if I really know the person, but if they're only a casual acquaintance anyway, I'll defriend for pretty much anything offensive with no regrets. If they're just someone I added a week ago because they messaged me and said "Hey, I was friends with your cousin in third grade," and I thought "Well, I barely remember them, but what the heck," and then they post something offensive, they're gone. It's only the people I care about who I'll bother to argue with on this kind of thing.

(ETA: Before this becomes a debate on the definition of "friend," I know not everyone uses Facebook the same way I do. I'm OK with having numerous casual acquaintances on my "friends" list and it doesn't bother me semantically. I see facebook "friend" as meaning "I want to read this person's posts," not "we are eternal bosom buddies.")

The Sandy Hook conspiracy theory is one of the most vile things created. If one of my relatives was endorsing it, or at least appeared to, I would want to know - but I don't think it should be done online. If my relative had been simply tricked by someone with a good line, I would hope to be able to put her straight. If she actually stuck by her theory, I would have to plan to avoid her whenever possible from that point on. It would not be something I would like to engage in in public.

The Sandy Hook conspiracy theory is one of the most vile things created. If one of my relatives was endorsing it, or at least appeared to, I would want to know - but I don't think it should be done online. If my relative had been simply tricked by someone with a good line, I would hope to be able to put her straight. If she actually stuck by her theory, I would have to plan to avoid her whenever possible from that point on. It would not be something I would like to engage in in public.

The Sandy Hook conspiracy theory is one of the most vile things created. If one of my relatives was endorsing it, or at least appeared to, I would want to know - but I don't think it should be done online. If my relative had been simply tricked by someone with a good line, I would hope to be able to put her straight. If she actually stuck by her theory, I would have to plan to avoid her whenever possible from that point on. It would not be something I would like to engage in in public.

This.

Actually, this is why I just hid the story and was waiting to see if niece posted anything else. Then I would have chatted with her. However, it seems like a one-off thing. So I am leaving it alone.

The Sandy Hook conspiracy theory is one of the most vile things created. If one of my relatives was endorsing it, or at least appeared to, I would want to know - but I don't think it should be done online. If my relative had been simply tricked by someone with a good line, I would hope to be able to put her straight. If she actually stuck by her theory, I would have to plan to avoid her whenever possible from that point on. It would not be something I would like to engage in in public.